Disinfection, Part 1: Developments in Ultraviolet Disinfection

Dec. 16, 2002
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Water treatment systems are designed to address public concerns such as taste, odor and hardness. The average person's greatest concern is microbiological safety; he wants to know that there isn't anything in his water that can make him ill. This concern explains why one of the fastest growing technologies in the water treatment industry is ultraviolet (UV) disinfection.

In an age of increased concern for personal health and protection of the environment, UV is an attractive alternative to chemical disinfectants. Chemical disinfectants often produce byproducts that can be harmful and may alter the taste of the water. UV disinfection adds no chemicals, and it does not produce byproducts. Additional benefits include easy installation, low maintenance, minimal space requirements and whole-house (point-of-entry) treatment.

Technology Improvements

There are several recent advances that make UV systems even more appealing today. One is a timer that monitors how long the system has been operating--this feature warns the user when his lamp is nearing the end of its 12-month life so he does not have to try to remember if it was one, two or three years ago that he bought his last lamp.

More efficient lamps are another recent development. These have allowed some manufacturers to produce systems that are half the size of conventional systems--an important consideration given that most systems are installed vertically in the basement.

UV Intensity Monitors

There is no better way to verify the real-time performance of a UV system than with a UV intensity monitor. The monitor is much like an "eye" looking into the reaction chamber. It measures how much light is getting through the water and often is calibrated to set off an alarm when sufficient UV light is not detected. On some systems, such a condition also can trigger a shut-off valve.

Whether it is changing water conditions, lamp fouling or lamp aging, a monitor assures that an adequate dose is delivered at all times.

UV monitors have been available for many years, however some older models tended to drift over time. Some manufacturers are now able to offer sensors that are much more resistant to the degradation caused by exposure to the UV light.

Reactor Efficiency

Pathogens traveling through a UV reaction chamber may follow a number of different paths. Some paths are longer, some are shorter, and some paths flow at faster velocities than others. The result is that the amount of UV light to which a pathogen is exposed can vary. To maximize the efficiency of a reactor, all the paths should expose pathogens to the same amount of UV light. Only by using sophisticated computational fluid dynamic (CFD) modeling can designers optimize a reactor. CFD modeling is the same process used to optimize car and airplane designs for air flow.

Figure 1 illustrates an efficient CFD-modeled reactor. Each colored strand represents the path of an individual particle through the reactor. The color changes from blue to red as the retention time increases.

For all its strengths, CFD is only a design tool and performance tests must still validate the effectiveness of each system. The best assurance of effectiveness is certification by a third-party testing body such as NSF International.

NSF Standard 55

NSF International is an independent accreditor of water treatment systems. The protocol for validation of residential UV systems is NSF Standard 55. The standard has two parts.

*               Class A is for UV equipment that is certified to be used for treatment of microbiologically unsafe water that meets all other drinking water criteria. The dose requirement is 40 mJ/cm2 for a municipality. Until very recently, there were not UV systems with Class A certification.

*               Class B is for UV equipment that is used to provide supplemental treatment of treated and disinfected public drinking water. The dose requirement is 16 mJ/cm2.

Regulators generally are interested in Class A certification because of the health effect claims that can be made by products receiving this certification. This includes 99.99 percent inactivation of Rotavirus. Whether viral, bacteriological or protozoan (such as Cryptosporidium parvum and Giardia or beaver fever) Class A certification assures that dangerous microorganisms are eliminated.

Systems that are certified and listed by NSF are the only systems that have been tested and have passed the Standard 55 Class A protocol. Do not be misled by statements that indicate the product "meets Standard 55 requirements." If a product is certified it will be listed on the NSF website (www.nsf.org/certified) under "Drinking Water Treatment Units," "Reduction Claims for UV Microbiological Water Treatment Systems" and "Disinfection Performance, Class A."

UV disinfection for residential use is growing rapidly in popularity. POE systems provide chemical-free disinfection, easy installation, minimal maintenance and proven protection against waterborne illnesses. There have been many advances in UV technology for the residential market. Water chamber and lamp efficiency, automatic timers, UV intensity monitors and NSF certification are some of the key features to consider when selecting the appropriate product for you and your customers.         

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